Prevent FENTRY_CALL reordering

FEntryInserter prepends FENTRY_CALL to the first basic block. In case
there are other instructions, PostRA Machine Instruction Scheduler can
move FENTRY_CALL call around. This actually occurs on SystemZ (see the
testcase). This is bad for the following reasons:

* FENTRY_CALL clobbers registers.
* Linux Kernel depends on whatever FENTRY_CALL expands to to be the very
  first instruction in the function.

Fix by adding isCall attribute to FENTRY_CALL, which prevents reordering
by making it a scheduling boundary for PostRA Machine Instruction
Scheduler.

Reviewed By: niravd

Differential Revision: https://reviews.llvm.org/D91218

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index fd658cd..900abd2 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -713,6 +713,7 @@ bool MachineInstr::isCandidateForCallSiteEntry(QueryType Type) const {
   case TargetOpcode::PATCHPOINT:
   case TargetOpcode::STACKMAP:
   case TargetOpcode::STATEPOINT:
+  case TargetOpcode::FENTRY_CALL:
     return false;
   }
   return true;